text: In addition to the amino acids, peptides and proteins bind metal cofactors through their side chains. For the most part, the α-amino and carboxylate groups are unavailable for binding as they are otherwise engaged in the peptide bond. The situation is more complicated for the N-terminal and O-terminal residues where the  α-amino and carboxylate groups are unavailable, respectively.  Especially important in this regard are histidine (imidazole), cysteine (thiolate), methionine (thioether).